Project Summary/Abstract Prostate cancer (PC) is the most common non-skin cancer in American men, with a lifetime incidence of 1 in 7, and also the second leading cause of cancer death in American men. Androgen receptor (AR) is the primary oncogenic driver of PC growth, survival and progression. AR-directed therapy is currently the principal treatment regimen. Despite initial response rates exceeding 90%, PC eventually relapses and progresses to fatal castration-resistant PC (CRPC), where reactivation of AR signaling occurs in a low-androgen environment. Recent introduction of FDA-approved next-generation antiandrogens, including enzalutamide (ENZ) and abiraterone acetate (ABI), have improved the CRPC treatment landscape, but emergence of drug resistance remains nearly universal, with no AR-targeted therapeutic options afterwards. These dismal facts underscore the pressing clinical need to identify new molecular targets and develop effective therapies to combat advanced PC. Through integrated analysis of publicly available clinical PC data sets coupled with functional studies in AR-positive PC cells, we propose monoamine oxidase A (MAOA), which synergizes with AR to promote PC, as an ideal therapeutic candidate to complement AR-targeted therapy in CRPC. We identified a novel reciprocal interaction between MAOA and AR in PC cells. MAOA expression is induced by androgen treatment; and conversely, MAOA silencing significantly reduces AR activity by lowering AR target gene expression and responsiveness to androgen stimulation in PC cells under both androgen-replete and depleted conditions as well as in a CRPC xenograft model. We showed significant positive co-expression of MAOA and AR target genes (PSA, TMPRSS2, NKX3.1) in multiple clinical data sets, including CRPC. Importantly, we found MAOA genomic amplification and/or epigenetic activation in 64% of samples in a CRPC data set, reinforced by elevated MAOA protein expression in our CRPC patient cohort. Additionally, we demonstrated that inhibition of MAOA by genetic or pharmacological approaches enhanced the growth-inhibiting effects of ENZ and ABI in androgen-sensitive, CR and antiandrogen-resistant PC cells. Based on these findings, we will test the hypothesis that MAOA synergizes with AR through reciprocal crosstalk and convergent downstream signaling to amply MAOA/AR effects promoting AR-driven PC growth and progression, and that co-targeting MAOA/AR is an actionable, effective strategy to treat CRPC and reverse antiandrogen drug resistance. To address this hypothesis, three aims are proposed. In Aim 1, we will elucidate the mechanistic basis of MAOA- AR reciprocal interaction in PC cells. In Aim 2, we will characterize the role of MAOA in regulating the development and progression of CRPC in xenograft models. In Aim 3, we will determine the efficacy of MAOA inhibitors for treating CRPC and reversing resistance to next-generation antiandrogens in vitro and in vivo. These studies will provide fundamental innovative insights into AR regulation in CRPC and illuminate a path toward the development of new combination therapy for advanced PC.